The escalating requirements for high density and performance associated with ultra large scale integration require responsive changes in conductive patterns, which is considered one of the most demanding aspects of ultra large scale integration technology. High density demands for ultra large scale integration semiconductor wiring require increasingly denser arrays with narrower conductive lines and reduced spacing between conductive lines. This objective becomes particularly difficult to achieve given the economic pressure for high speed production with existing equipment. Thus, the combined requirements of high speed and high density conductive wiring patterns pose a challenge which, to date, has not been satisfactorily met.
The ever increasing demands for semiconductor devices containing conductive patterns having increasingly narrower line widths and increasingly narrower interwiring spacing therebetween generate acute problems particularly with respect to current photolithographic capabilities. For example, photolithographic production equipment, called i-line steppers, employs an ultra-violet (UV) source having a wave length of about 0.365 .mu.m. However, as the requirement for conductive line widths and interwiring spacing decreases to below about 0.5 .mu.m, particularly below 0.40 .mu.m, such i-line stepper equipment is pressed to the limit of its capability. The considerable investment in such i-line stepper equipment creates an economic incentive to satisfy the ever increasing demands for smaller interwiring spacing and narrower conductive lines without replacing existing i-line stepper equipment.
A conventional method of forming a pattern of conductive lines comprises a subtractive etch back step as the primary metal-patterning technique. This traditional technique basically comprises forming an insulating layer, as on a semiconductor substrate, typically monocrystalline silicon, and depositing a conductive layer on the insulating layer. The conductive layer can comprise aluminum, an aluminum alloy, tungsten, polysilicon, tungsten silicide, or titanium silicide. A photoresist mask is then formed on the conductive layer, by conventional photolithographic techniques. The photoresist mask defines a pattern of conductive lines for substantial reproduction in the conductive layer. The conductive pattern is then formed in the conductive layer by etching through the photoresist mask.
Typically, etching is conducted to optimize production speed, as by utilizing an etching apparatus which generates a high density plasma, e.g., a high density chlorine plasma. Such a high density plasma etching technique comprises feeding chlorine gas, along with boron trichloride, into an etching apparatus, such as a Transformer Coupled Plasma (TCP) source type of apparatus to generate a high density plasma with Cl.sup.- as the etching species. Model 9600, commercially available from Lam Research Corp., Fremont, Calif., has been found suitable. Other types of etching apparatus, such as other high density plasma source types of apparatus, can be used. For example, etching can also be conducted with an Electron Cycletron Resonance (ECR) type apparatus, a Helicon Resonant Inductive coupled plasma source type apparatus or a Decoupled Plasma Source (DPS) type of apparatus.
Etching is normally conducted until the conductive material, typically a metal, is substantially removed between the metal lines along with any residues which may have formed. Overetching is conventionally performed to remove a portion of the underlying oxide to ensure complete removal of products between the metal lines.
In order to efficiently manufacture semiconductor devices having a conductive pattern with accurately and reproducibly formed conductive lines having a width and interwiring spacing of about 0.5 .mu.m or less, particularly below 0.40 .mu.m, it is necessary to employ a photoresist mask which is very thin to ensure an adequate process margin, i.e., an adequate depth of focus. Unfortunately, the etch selectivity to conventional photoresist materials when etching an underlying conductive layer, such as a metal, is not sufficiently high to avoid lateral etching of the side surfaces of the pattern defined in the photoresist mask. Consequently, during etching of the underlying metal to form a conductive pattern, as by high density plasma etching, the profile of the photoresist mask pattern is disadvantageously altered resulting in undercutting of the metal lines with a consequential reduction in the width of the metal lines. Any such reduction in the width of a metal line below the design rule significantly affects line integrity and, hence, the performance of the resulting semiconductor device. Thus, in order to accurately and reproducibly form conductive patterns with sub-half micron geometry employing conventional i-line stepper equipment, new approaches are required.
A conventional etching process to form a conductive pattern is illustrated in FIGS. 1-3. As shown in FIG. 1, insulating layer 10, such as an oxide layer, e.g., silicon dioxide, is formed as on a semiconductor substrate (not shown), and a conductive layer is formed thereon. The conductive layer depicted in FIG. 1 is a composite conductive layer comprising barrier metal layer 11, such as titanium, a primary metal layer 12, such as aluminum, and anti-reflective coating 13, such as titanium nitride. A photoresist mask 14 is formed on the composite conductive layer by conventional photolithographic techniques. The photoresist mask comprises an organic material and defines a pattern of lines to be formed in the composite conductive layer.
As shown in FIG. 2, the inadequate etch selectivity between photoresist mask 14 and the composite conductive layer comprising barrier metal layer 11, conductive metal layer 12 and anti-reflective coating 13, results in etching of the side surfaces of photoresist mask 14 altering its profile. It should be recognized that in FIGS. 1-7, similar elements bear similar reference numerals. Upon etching the composite conductive layer, the etch end point is detected and overetching is conducted to ensure complete removal of any reaction products and minimize bridging between the metal lines. As shown in FIG. 2, the profiled side surfaces of photoresist mask 14 undesirably cause a reduction of the width of the etched metal lines. This problem is exacerbated when employing relative thin photoresist masks, such as below about one micron for etching a composite conductive layer of 0.8 microns or thicker.
Hard masks have been proposed for use in defining a conductive pattern, such as a mask made of silicon dioxide, silicon nitride or silicon oxynitride. However, the use of a hard mask increases process complexity and, hence, increases the expected defect level. Moreover, the use of a hard mask increases manufacturing costs.
Nitrogen has previously been included in etch chemistry to improve etch selectivity with respect to a resist during etching and to provide slightly sloped metal profiles. The amount of nitrogen incorporated has been typically limited to less than 10% by volume of the total gas flow. Alternatively, nitrogen is employed only during the early etch sequence and cut off, in order to minimize nitrogen induced residues.
In copending application Ser. No. 08/359,232 filed on Dec. 19, 1994, a method is disclosed for etching a composite comprising a photoresist mask on an anti-reflective coating having a non-planar topography. In accordance with the disclosed method, the composite is etched using an inert gaseous nitrogen plasma to maintain the resist profile by increasing the selectivity of the anti-reflective coating vis-a-vis the photoresist during removal of a spin-on anti-reflective layer.
In copending application Ser. No. 08/554,413 filed on Nov. 8, 1995, a method is disclosed wherein an inert gas plasma, preferably containing nitrogen, is employed to etch an anti-reflective coating without any substantial etching of exposed underlying polysilicon.
In copending application Ser. No. 08/657,261 filed on Jun. 3, 1996, the metal etch rate of a conductive material within a dense array of conductive lines is increased by injecting nitrogen into the gas flow of the plasma during high density plasma etching.
There exist a need for semiconductor technology enabling etching a conductive layer, as by high density plasma etching, to form sub-half micron conductive line patterns without encountering a reduction in line width due to etching of the photoresist mask.